Black is usually a sign of tissue death and necrosis. Perhaps it was caught under something or stepped on? It may fall off eventually or self amputate, as happens in crush or frostbite injuries. You might want to apply Betadine or soak the foot in warm Epsom salts.
Pen̈icillan and cephaxion work together. Betadine soaks will help scrape off the gangrene dermatitas they can get if they drink bannix or sprays in water